Kilns in which mineral matter is subjected to burning are commonly fired by means of a so-called fossil fuel, such as pulverised coal, natural gas or fuel oil, or a fuel derived therefrom, such as coal gas. However, the combustion of such a fuel in the presence of a combustion-supporting gas, usually air, may give rise to the generation of nitrogen oxides (NO.sub.x), the emission of which into the atmosphere is undesirable for environmental reasons. Such nitrogen oxides are generated from nitrogen compounds that are present in the fuel; also, if the combustion temperature is sufficiently high, in particular above 1200.degree. C., nitrogen oxides would also be generated by the reaction of nitrogen and oxygen that are present in the combustion-supporting air. The patent literature contains a number of proposals for ways in which the emission of NO.sub.x from mineral-burning processes may be reduced, for example as disclosed in the following documents.
DE-A-3 426 296 discloses a process for reducing the content of NO.sub.x in the exhaust gas of a rotary kiln in the manufacture of cement, in which air that has been heated during the cooling of the cement clinker is passed, a combustion air, through the kiln, a calciner and a cyclone preheater in counter-current to the cement raw material. The firing in the calciner is reducing in a first zone but, on account of supply of heated combustion air, is oxidising in a subsequent zone. Part of the gas leaving the kiln is passed through the reducing zone in the calciner, in which nitrogen oxide is reduced to nitrogen and fresh formation of nitrogen oxide is very largely obviated. Thereafter, this part of the gas is mixed with the other part of the gas leaving the kiln and the mixture is passed through the oxidising zone of the calciner. The said other part of the gas, on account of its content of oxygen, will enable the carbon monoxide, which is formed in the reducing zone, to be completely burnt. A similar method is disclosed for processes in which the air is passed through the kiln and then through a grid preheater.
U.S. Pat. No. 4,808,108 discloses a method of heat-treating fine-grained material, in which the material is preheated in a preheating zone using the hot exhaust gases from a precalcination zone, the preheated material is then further heated in the precalcination zone by the combustion of additional fuel, and the material subsequently undergoes burning in a combustion zone, in particular a rotary kiln, and the exhaust gases from the combustion zone are passed into the precalcination zone. The fuel that is combusted in the precalcination zone is delivered at two locations that are spaced one from the other. The burnt material that issues from the combustion zone may be cooled in a cooler by means of air and the air which is exhausted from the cooler may then be passed into the precalcination zone at two spaced locations, each adjacent a respective location at which the fuel enters the precalcination zone. The portion of the fuel that is added to the precalcination zone at the point nearer to the rotary kiln and the content of oxygen in the gases in this region are controlled so as to achieve a sub-stoichiometric combustion resulting in the establishment of a carbon monoxide content between 0.05 and 1 per cent. Under these reducing conditions, the nitrogen oxides contained in the exhaust gases from the rotary kiln, which are passed into the precalcination zone, are largely decomposed to nitrogen, with the formation of carbon dioxide. The material introduced into the precalcination zone from the preheating zone has a catalytic effect on the removal of the nitrogen oxides and also serves to regulate the temperature since the deacidification thereof takes up the heat produced by the combustion. In the other region where fuel is introduced into the precalcination zone, the combustion of the fuel also gives rise to the combustion of the carbon monoxide emerging from the region of the precalcination zone adjacent the kiln.
EP-A-0 391 463 discloses a method for the reduction of NO.sub.x emission from apparatus for burning mineral raw materials, especially cement raw materials, by means of a nitrogen-containing solid fuel such as coal. The apparatus comprises a burning zone, in particular a rotary kiln, a precalcining zone for calcining the raw materials prior to their entry into the burning zone, and a multi-string, multi-stage preheater system for preheating the raw materials by heat exchange with the exhaust gases from the burning zone and/or from the precalcining zone. One preheater string, the kiln string, is in direct connection with a kiln riser pipe and another preheater string, the calciner string, is connected to the precalcining zone. Material from the burning zone is cooled in a cooling zone, at least some of the exhaust air from which is passed to the precalcining zone as combustion air. Preheated raw material from the kiln string and the calciner string is subjected to precalcination in the precalcining zone and thereafter the bulk of the precalcined raw materials is fed to the burning zone. A nitrogen-containing solid fuel such as coal is fed into the kiln riser pipe in an amount as to constitute from 20 to 50 per cent of the total amount of solid fuel supplied to the apparatus. The nitrogen introduced into the kiln riser pipe by means of the fuel reacts with the nitrogen oxide (NO.sub.x generated in the burning zone and present in the exhaust gas therefrom, thereby forming harmless nitrogen (N.sub.2). The non-combusted fuel, which now contains less nitrogen, is separated from the gases in the lowermost preheater stage of the kiln string and is conveyed to and combusted in the calcining zone.
WO-A-97/30003 discloses a method for reducing the NO.sub.x emission from a kiln plant for the heat treatment of raw materials, in which plant nitrogen-containing fuel can be fired in at least three different zones. In a first such zone, in particular a kiln, an amount c of fuel is burned and to this zone there is fed an oxygen-containing gas. In a second such zone, especially a calciner, an amount b of fuel is burned and to this zone there is fed NO-containing exhaust gas from the other two zones, the exhaust gases from the said second zone being removed from the kiln plant. In the third such zone, in particular a burning compartment, an amount a of fuel is burned and to this zone there is fed at least a portion of the raw materials and also an oxygen-containing gas. The oxygen-containing gas fed to the said second and third zones may be exhaust air from a cooler in which the burned material from the kiln is cooled. The amount of fuel b+a is determined by the requirements of the raw-material treatment; also, the amounts of fuel b and a are adjusted until a minimum NO content is achieved in the exhaust gases from the said second zone. The temperatures in the second and third zones should be as high as the process permits but below 1200.degree. C. In certain embodiments the temperature in the second zone is from 900 to 1150.degree. C. and the temperature in the third zone is from 1000 to 1200.degree. C.
There have also been proposals for the disposal of waste materials, for example used tyres, in kilns and/or associated plant for the heat treatment of mineral matter, in particular kilns and/or associated plant for the production of cement.
Every year large numbers of pneumatic tyres, in particular automobile tyres, are discarded. For example, it was reported in Rock Products, October 1980, that over 200 million automobile tyres were being discarded every year in the United States, and that some 300,000 tons of old tyres were being generated annually in what was then West Germany. A large proportion of waste tyres are simply buried in land-fill sites and there have been various proposals for using comminuted waste tyres, e.g. in road-building asphalts; nonetheless, the environmentally acceptable disposal of waste tyres still presents a considerable challenge.
It has been recognised that the calorific value of waste tyres, at 27,000 kJ/kg, is comparable to that of coal and it is known to use chipped or otherwise comminuted tyres as an auxiliary fuel in cement kilns. However, when account is taken of the capital investment needed for machines for chipping the tyres and the energy costs in operating such machines, the cost benefits obtained by replacing coal or other fossil fuel with tyre chips are often not substantial. Furthermore, if the tyre chips are to be fed into a precalciner or preheater system, it is necessary carefully to control the velocity of the gas in that system to ensure adequate combustion of the chips before they are swept into the next stage of the preheater.
There have been proposals for feeding whole tyres directly into a rotary cement kiln. For example, U.S. Pat. No. 4,551,051 discloses apparatus by means of which whole tyres may be delivered through the mineral-inlet end of a rotary cement kiln to a selected region of the kiln. In U.S. Pat. No. 5,078,594 an apparatus is described for charging whole tyres through a port in the wall of a rotating cement kiln into a heated-zone of the kiln.
However, the delivery of whole tyres directly into a rotary cement kiln requires careful control to ensure that the tyres are fully burnt within the kiln, that there is no localised reduction, which can lead to the build-up of solids and blockages in the lower stages of the preheater system, and that the solid residue is uniformly incorporated into the cement clinker.
DE-A-3,326,375 discloses a method for the production of cement clinker in a rotary kiln, in which the raw material is preheated in a multi-stage cyclone preheater and is then calcined using different fuel components in at least two different places in the preheater before entry into the rotary kiln. This method is characterised in that the raw material is preheated and calcined in two separate streams in separate strings of the preheater, in that a coarse particulate solid fuel component (for example used tyres or lump coal) is supplied to the first preheater string at a point located close to the rotary kiln such that this fuel component arrives in the rotary kiln before complete combustion; and in that a liquid, gaseous of fine particulate solid fuel component is introduced into the second string of the preheater such that this fuel component is thoroughly mixed with the material to be calcined before complete combustion. It is suggested in this German patent application that this method permits the use of quite different types of fuel without the formation of coatings or other operational problems in the preheater; it is suggested that this is due to the fact that the coarse particulate fuel component is practically completely gasified in the rotary kiln (page 5, lines 1-6, and page 5, line 25 to page 6, line 1). In the detailed description of an illustrated embodiment, the point at which the coarse particulate solid fuel component is supplied to the first preheater string is selected such that the coarse particulate fuel will fall more or less freely through the inlet housing associated with the rotary kiln and thence into the rotary kiln itself, where it is gasified (page 11, lines 1-9). There is no disclosure or suggestion that the coarse particulate fuel is maintained in contact with the hot gas stream in the preheater.
U.S. Pat. No. 4,295,823 and U.S. Pat. No. 4,627,877 describe, and respectively claim, an apparatus and a method for continuously producing a cement clinker in which a combustible waste material is employed as a heat source. According to the disclosure in these U.S. patents, a cement raw material is fed into a preheating or calcining chamber, from which the resultant preheated or calcined material is forwarded into a heating region within a rotary kiln in order to convert the material into a cement clinker. The cement clinker is then moved into a cooling chamber in which its temperature is decreased by means of cooling air. Exhaust gas from the heating region in the rotary kiln is passed into the preheating or calcining chamber. The combustible waste material is fed into a heat-decomposition chamber into which exhaust gas from the heating region of the rotary kiln is also introduced and in which the combustible waste material is thermally decomposed in order to generate a combustible gas. This combustible gas is then conveyed to, and burnt within, the preheating or calcining chamber to preheat or calcine the cement material. Accordingly, the said waste material undergoes a pyrolysis and gasification process rather than combustion as such. Any solid residue from the thermal decomposition of the combustible waste material is allowed to pass into the heating region within the rotary kiln. Various combustible waste materials are mentioned, including tyre wastes, rubber wastes, oil wastes, oil-containing sludges, asphalt wastes, pitch wastes, and organic compound wastes.
U.S. Pat. No. 5,816,795 discloses a preheater or precalciner cement kiln including a rotary vessel, a riser duct for counterflow preheating of cement raw materials, and a shelf-transition portion between the riser duct and the rotary vessel for receiving preheated mineral material from the riser duct and delivering it into the rotary vessel. The riser duct is modified in order to permit the burning in the kiln gas stream of solid fuel, in particular combustible waste material. A solid fuel delivery port is formed in the riser duct at a point downstream of the self-transition portion (relative to the kiln gas flow). A fuel delivery tube extends through the fuel delivery port and includes a fuel inlet end that is outside the riser duct and a fuel outlet end that is inside the riser duct. A grate is positioned in order to receive and suspend solid fuel emerging from the output end of the fuel delivery tube in the riser duct. A feed mechanism, for example a reciprocating ram, is used to feed the fuel through the delivery tube and onto the grate. The fuel delivery tube may comprise an intermediate staging portion between the inlet and outlet ends, which staging portion and the outlet end are preferably insulated. In certain embodiments insulation is provided by an annular passageway for the delivery of oxygen-containing gas such that the gas contacts the solid fuel suspended on the grate in the kiln gas stream. A combustion control agent, for example cement raw material or cement kiln dust, may be metered into the intermediate staging portion so as to contact the fuel therein which is awaiting delivery onto the grate.
There remains a need for an improved method whereby the content of NO.sub.x in the effluent gas issuing from a mineral-burning kiln can be substantially reduced. There also remains a need for an improved method whereby whole tyres can be disposed of in a mineral-burning, e.g. cement-manufacturing, process.